This invention relates to chemical vapor deposition methods for providing a Bi oxide-containing film on a surface of a substrate by decomposing a precursor of Bi oxide.
Interest in ferroelectric has increased in recent years, due to the utility of these materials in applications such as non-volatile memories. Information in these memories is stored by the polarization of a thin ferroelectric film which is placed between the two plates of a capacitor. The capacitor is connected to a transistor to form a storage cell, which controls the access of read-out electronics to the capacitor.
The information stored in the cell can be changed by applying an electric field to the thin ferroelectric film and flipping the polarization. Ferroelectric random access memories (FERAMs), unlike DRAMs (dynamic random access memories), retain the stored information if the power supply is turned off. In addition, they do not require refresh cycles. Desirable electrical properties for ferroelectrics used in memory applications include: (a) a low coercive field, which makes the use of as low a voltage supply as possible; (b) a high remanent polarization, which is needed for high reliability of information storage; (c) minimal fatigue, which is required for a long life-time; and (d) no imprint, as an imprint would alter the stored information.
Strontium bismuth tantalate (SrBi2Ta2O9) (SBT) is a ferroelectric material that meets all of these requirements. Significant efforts are therefore being made to integrate this material into memory devices. Capacitors in which SBT is incorporated using a sol-gel method have good electrical properties. The sol-gel method provides only a low integration density of SBT, however. To achieve a higher integration density of SBT, an alternative method, such as chemical vapor deposition (CVD), must be used.
In one aspect, the invention features a method of forming a Bi-containing metal oxide film on a substrate, by dissolving a precursor of Bi oxide in a solution, decomposing the precursor to form Bi oxide, and depositing the Bi oxide on the substrate at a temperature lower than 450xc2x0 C. Bi complexes which include at least one alkoxide group are used as the precursor of Bi oxide.
Embodiments of this aspect of the invention may include one or more of the following features.
The deposition temperature may be lower than 400xc2x0 C. The Bi oxide-containing film may also be provided by adding the steps of decomposing a precursor of Sr oxide, and a precursor of Ta oxide to form Sr oxide and Ta oxide, respectively, and depositing the Bi oxide, the Sr oxide and the Ta oxide on the substrate.
The film of Bi, Sr, and Ta oxides may be deposited as a ferroelectric film or can be converted into a ferroelectric film by an annealing process.
In accordance with the invention, the ferroelectric layer is formed from an amorphous as-deposited layer or film. The amorphous film is annealed, transforming it into the ferroelectric layer. We have discovered that by forming the ferroelectric layer from an amorphous layer, a lower thermal budget is consumed by the ferro-anneal as compared to that of ferroelectric layer formed from conventional techniques. The lower thermal budget avoids or reduces excessive diffusion of one or more of the constituents of the ferroelectric layer and oxidation of the contacts.
The amorphous layer is processed to produce a ferroelectric layer in accordance with the invention. The amorphous layer comprises materials that can be transformed into a ferroelectric layer. In one embodiment, the amorphous layer comprises a Bi-based oxide ceramic. The Bi-based oxide ceramic comprises, for example, strontium bismuth tantalate (SBT) or a material derived from SBT (SBT derivative). The amorphous layer is annealed under appropriate conditions transforming it into a ferroelectric layer.
The amorphous film comprises a material which can be processed, for example by a ferro-anneal, to form a ferroelectric film. In one embodiment, the amorphous layer comprises a metal oxide ceramic material. Preferably, the amorphous layer comprises a Bi-based oxide ceramic material. More preferably, the amorphous layer comprises a Bi-based oxide ceramic material that can be processed to transform it into a ferroelectric.
The Bi-containing metal oxide film is formed by placing the substrate in a CVD chamber, heating the substrate to a deposition temperature lower than 450xc2x0 C., introducing vapors of the precursors of Bi, Sr, and Ta oxides to the CVD chamber, decomposing the precursors of Bi, Sr, and Ta oxides, and depositing the oxides on the substrate. Precursors of Bi, Sr, and Ta oxides may be decomposed in the presence of an oxidizer by oxidative decomposition, where examples of the oxidizers include O2, singlet O2, O3, H2O2, N2O, NOx (1xe2x89xa6xxe2x89xa63), and downstream oxygen plasma, and where the concentration of the oxidizer is between 5% and 95% of the total gas and vapor flow into the CVD chamber. At least one of O2 and N2O may be used as the oxidizer. The oxidizer may be formed in the CVD chamber by converting an oxidizer molecule into an active oxidizer by applying to the CVD chamber plasma, UV light, heat, a sensitizer, or ion beams.
The precursor of Bi oxide may have the formula Bi(OR)3, Bi(OR)2(ORxe2x80x2), or Bi(OR)(ORxe2x80x2)(ORxe2x80x3), where each of R, Rxe2x80x2, and Rxe2x80x3 is, independently, an alkyl, aryl, or silyl group. For example, R may be tpentyl, pentyl, tBu, Bu, iPr, Pr, Et, Me, Ph, aryl, or SiRxe2x80x2xe2x80x33, and Rxe2x80x2xe2x80x3 may be tBu, Bu, iPr, Pr, Et, or Me. Examples of precursors of Bi oxide further include Bi(OtBu)3 and Bi(OCMe2Et)3. The precursor of Bi oxide may also include an alkoxy group, a phenoxy group, or a donor atom such as N, O, or S. For example, the precursor may include the group xe2x80x94CH2CH2xe2x80x94N(CH3)2.
The Bi-containing metal oxide deposited on the substrate may have the formula (Bi2O2)2+(Amxe2x88x921BmO3m+1)2xe2x88x92, where A is Bi3+, L3+, L2+, Ca2+, Sr2+, Ba2+, Pb2+, or Na+, B is Fe3+, Al3+, Sc3+, Y3+, L3+, L4+, Ti4+, Nb5+, Ta5+, W6+, or Mo6+, and L is Ce4+, La3+, Pr3+, Ho3+, Eu2+, or Yb2+, and where 1xe2x89xa6mxe2x89xa65. The Bi-containing metal oxide may also have the formula Bi2WO6; BiMO3, where M is Fe or Mn; Ba2BiMO6, where M is V, Nb or Ta; Pb2BiMO6, where M is V, Nb or Ta; Ba3Bi2MO9, where M is Mo or W; Pb3Bi2MO9, where M is Mo or W; Ba6BiMO18, where M is Mo or W; Pb6BiMO18, where M is Mo or W; KBiTi2O6; or K2BiNb5O15. These metal oxides can be obtained by decomposing precursors which contain the above-described metals.
The Bi-containing metal oxide film can also be a SBT derivative. Examples of such derivatives include SrBi2Ta2O9; SrBi2Ta2xe2x88x92xNbxO9, where 0xe2x89xa6xxe2x89xa62; SrBi2Nb2O9; Sr1xe2x88x92xBaxBi2Ta2xe2x88x92yNbyO9, where 0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa62; Sr1xe2x88x92xCaxBi2Ta2xe2x88x92yNbyO9 where 0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa62; Sr1xe2x88x92xPbxBi2Ta2xe2x88x92yNbyO9, where 0xe2x89xa6xxe2x89xa61 and 0xe2x89xa6yxe2x89xa62; and Sr1xe2x88x92xxe2x88x92yxe2x88x92zBaxCayPbzBi2Ta2xe2x88x92pNbpO9, where 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61, and 0xe2x89xa6pxe2x89xa62. An element of the metal oxide may be substituted by a metal such as Ce, La, Pr, Ho, Eu, and Yb.
The precursor of Sr oxide generally has the formula Sr(thd)2 or Sr(thd)2 adduct, and may include a polyether or a polyamine. The polyether has the formula Rxe2x80x94Oxe2x80x94(CH2CH2O)nxe2x80x94Rxe2x80x2, where 2xe2x89xa6nxe2x89xa66, and where each of R and Rxe2x80x2 may be, independently, an alkyl group, an aryl group, or hydrogen. The polyamine has the formula Rxe2x80x94NRxe2x80x3-(CH2CH2NRxe2x80x3)n-Rxe2x80x2, where 2xe2x89xa6nxe2x89xa66, where each of R and Rxe2x80x2 may be, independently, an alkyl group, an aryl group, or hydrogen, and where Rxe2x80x3 is H, Me, Et, or Pr. The precursor of Sr oxide may also include tetraglyme, triglyme, N,N,Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-pentamethyl-diethylene-triamine, or N,N,Nxe2x80x2,Nxe2x80x3,Nxe2x80x2xe2x80x3,Nxe2x80x2xe2x80x3-hexamethyl-triethylene-tetramine. The precursor of Sr oxide may also be Sr alkoxide, SR alkoxide mixed with Ta and Nb alkoxides, or a Lewis base adduct of the alkoxide, where the Lewis base is tetraglyme, triglyme, N,N,Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-pentamethyl-diethylene-triamine, or N,N,Nxe2x80x2,Nxe2x80x3,Nxe2x80x2xe2x80x3,Nxe2x80x2xe2x80x3-hexamethyl-triethyleneetramine.
The precursor of Ta oxide generally has the formula Ta(OR)5xe2x88x92n(X)n, where R is Me, Et, Pr, iPr, Bu, iBu, tBu, pentyl, or ipentyl, where X is xcex2-diketonate, and where 1xe2x89xa6nxe2x89xa65. For example precursor may be Ta(OiPr)4(thd). The precursor of Ta oxide may be an alkoxide including Ta pentakis (ethoxide), Ta pentakis (ipropoxide), Ta pentakis (tbutoxide), or Ta pentakis (tpentoxide).
The precursors of the Bi, Sr, and Ta oxides are dissolved in a solution of an aliphatic, a cycloaliphatic, or an aromatic solvent that may include a functional group such as an alcohol, ether, ester, amine, ketone, or aldehyde group. For example, the precursors of Bi, Sr, and Ta oxides may be dissolved in a solvent such as an alcohol. For example, t-butanol may be used as a solvent for Bi(OtBu)3, and t-pentanol for Bi(OCMe2Et)3. Alternatively, the precursors may be dissolved in a mixture of THF, iPrOH, and a Lewis base in a ratio of about 8:2:1, respectively, or a mixture of octane, decane, and pentamethyl-diethylene-triamine in a ratio of about 5:4:1. Furthermore, the precursor of Bi oxide may be dissolved in a solution comprising Lewis base adducts.
The solutions containing the precursors are evaporated by vaporizers. For example, the solution containing the precursor of Bi oxide is evaporated at a temperature from 130xc2x0 C. to 220xc2x0 C., and the solution for the precursors of Sr and Ta oxides is evaporated at a temperature from 170xc2x0 C. to 240xc2x0 C. An inert gas such as Ar, He, or N2 is added to the vapors of the solution, and a mixture of the inert gas and vapors is delivered to the CVD chamber. For example, the mixture includes vapors of the precursors of Bi oxide, Sr oxide, and Ta oxide in a ratio of about 2:1:2. It is appreciated that the concentrations of the precursors in the vapor mixture depend on several factors, including vaporization temperature, pressure in the vaporizer, gas and vapor flow rate, desired film stoichiometry, and geometry of the CVD chamber.
In the CVD chamber, the substrate is heated to the deposition temperature of 300xc2x0 C. to 450xc2x0 C. The pressure in the CVD chamber is maintained between 0.001 torr and 760 torr, for example, between 0.1 torr and 10 torr. An additional inert gas is added to the CVD chamber, where the concentration of the inert gas may vary from 10% to 90% of the total gas and vapor flow into the CVD chamber, for example, 30% to 50%. Preferably, the vapors of the precursors, the oxidizers, and an inert gas are introduced to the CVD chamber at a total flow rate of 1 ml/min to 15,000 ml/min, measured at the standard condition. The desirable flow rate may also depend on the temperature and the pressure of the gas and vapor mixture, desired film stoichiometry, and geometry of the CVD chamber. The oxides are deposited onto the substrate over a time period between 2 minutes and 2 hours, for example, between 2 minutes and 15 minutes. After deposition, the film is heated to a temperature of 600xc2x0 C. to 800xc2x0 C. for a time period between 5 minutes and 3 hours.
The substrate preferably includes Si, n-doped Si, n-doped Si, SiO2, Si3N4, GaAs, MgO, Al2O3, ZrO2, SrTiO3, BaTiO3, or PbTiO3. The film of Bi-containing metal oxide is deposited on a bottom electrode disposed on the substrate which includes a transistor. The bottom electrode is connected to the transistor by a plug. The bottom electrode may include a metal such as Pt, Pd, Au, Ir, or Rh; a conducting metal oxide such as IrOx, RhOx, RuOx, OsOx, ReOx, or WOx, where 0xe2x89xa6xxe2x89xa62; a conducting metal nitride such as TiNx, ZrNx, or WNyTaNy, where 0xe2x89xa6xxe2x89xa61.0 and 0xe2x89xa6yxe2x89xa61.7; or a superconducting oxide such as YBa2Cu3O7xe2x88x92x where 0xe2x89xa6xxe2x89xa61, and Bi2Sr2Ca2Cu3O10. The bottom electrode may be a Pt electrode.
A first intermediate layer may be provided between the bottom electrode and the plug. Examples of the first intermediate layer include a Ti adhesion layer and a Ti nitride diffusion barrier layer. A second intermediate layer may also be provided between the bottom electrode and the metal oxide layer. Examples of the second intermediate layer include a seed layer, a conducting layer, and a dielectric layer of high permittivity. The plug may include W or Si, and is connected to the bottom electrode and to a source/drain of a MOS field effect transistor. The film may also be used as a thin ferroelectric film for a ferroelectric capacitor, a ferroelectric memory, and/or a ferroelectric field effect transistor, for example, a metal ferroelectric semiconductor and a metal ferroelectric insulating semiconductor.
The substrate may be flushed with a mixture of an inert gas and the oxidizer before and/or after being exposed to the vapors of the precursors of the metal oxides. The processes of heating, decomposing, and depositing may be performed at least twice on the substrate. The substrate may also be removed from the chamber, treated by at least one intermediate process, such as a rapid thermal process, and returned to the chamber.
The operating conditions of the CVD may also be changed. For example, the compositions of the precursors, oxidizers, and inert gas in the mixture may be varied while the substrate is positioned in the chamber. Deposition temperature as well as the chamber pressure may also be varied. The precursor of Bi oxide may be delivered to the CVD chamber during a period between the onset of deposition and 30 minutes thereafter; the concentration of the Bi oxide is then decreased. In other methods, the substrate may be heated inside the chamber at a temperature lower than 450xc2x0 C. at least twice, or the substrate may be heated inside the chamber at a temperature lower than 450xc2x0 C. in the presence of at least one of the oxidizers O2 and O3.
In another aspect, the invention features a method of forming a metal oxide film on a substrate, by heating the substrate to a temperature lower than 450xc2x0 C. and introducing vapors of a precursor of Bi oxide to the substrate. Bi complexes which include at least one alkoxide group are used as the precursors of Bi oxide. The precursor of Bi oxide decomposes at the surface of the substrate to form Bi oxide, which is deposited on the surface of the substrate.
As used herein, the term xe2x80x9cprecursor of Bi oxidexe2x80x9d means any Bi complex which may be degraded to form Bi oxide. Examples of precursors of Bi oxide include Bi alkoxides, which have the structure Bi(OR)3, Bi(OR)2(ORxe2x80x2) or Bi(OR)(ORxe2x80x2)(ORxe2x80x3), where each of R, Rxe2x80x2, and Rxe2x80x3 is, independently, an alkyl or an aryl group. Bi alkoxides also include derivatives of the above-described precursors.
The use of Bi alkoxides as the precursors of Bi oxide in chemical vapor deposition offers numerous advantages. Bi alkoxides contain Bixe2x80x94O bonds which are relatively easy to cleave. Accordingly, Bi alkoxides can be decomposed at lower temperatures. Decomposition and deposition at a lower temperature decreases the migration of Bi oxide to the bottom electrode and the substrate. The degradation of the preexisting structure is thereby minimized. Furthermore, Bi alkoxides do not require oxygen as a co-reactant for the formation of the Bi oxide layer. It is believed that this contributes to conformal deposition of Bi oxides.
Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to these described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. In case of conflict, the present specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.